The invention relates generally to the field of electronic photography, and in particular to a digital camera capable of interfacing with a computer.
A tethered digital camera includes a CCD (Charge Coupled Device) image sensor, an A/D (Analog-to-Digital) converter, a line store, and a digital interface which connects to a host computer via a shared bus, such as a universal serial bus (USB). The digital camera provides variable zoom, resolution, and frame rate options, which are set by the host computer. The host computer also determines which CCD image sensor pixel data is transferred from the camera to the host computer. In the telephoto zoom setting, only the central part of the image sensor data is transferred to the host computer. However, all of the lines of image sensor data must be clocked out of the CCD""s vertical shift registers, even the xe2x80x9cextraxe2x80x9d lines at the top and bottom of the image sensor, which are not transferred to the host computer. Unfortunately, it takes time to clock out these xe2x80x9cextraxe2x80x9d lines, which reduces the maximum frame rate that can be achieved.
U.S. Pat. No. 5,668,597 describes an interline CCD sensor that reads out only pairs of lines in the central part of the image. Frame rate is not improved, however, since a succeeding image exposure does not begin until the vertical CCD register is completely cleared. Although scanned, MOS type sensors allow any arbitrary area of the image sensor to be read out, thus allowing the frame rate of a zoomed image to be faster, MOS sensors do not have vertical CCD storage registers so they do not have the problem of clearing these registers in the zoom mode.
U.S. Pat. No. 5,500,674, issued Mar. 19, 1996, inventors Takase et al. discloses a method of driving a solid-state image sensor having an effective pixel area formed of a plurality of pixels in an array having M pixels in the vertical direction and N pixels in the horizontal direction. A scanning pixel area formed of m pixels in the vertical direction and n pixels in the horizontal direction (where, M greater than m, Nxe2x89xa7n) is established within the effective pixel area. Charges stored in the pixels in the scanning pixel area are transferred by vertical CCDs driven by a first scanning pulse to a horizontal CCD. The charges transferred to the horizontal CCD are transferred by the horizontal CCD driven by a second scanning pulse to an output portion of the horizontal CCD. Further, while the driving of the horizontal CCD is stopped, the vertical CCDs are driven by a third scanning pulse having a higher frequency than the frequency of the first scanning pulse so that charges stored in the pixels in the effective pixel area outside the scanning pixel area are transferred at high speed to the horizontal CCD of which driving is stopped. The charges transferred from the vertical CCDs to the horizontal CCD at high speed are swept out of the horizontal CCD. This method has the disadvantage of reading out all of the pixels of one image before a succeeding image can be exposed.
This invention is directed to a method for improving the frame rate of the CCD image sensor.
The present invention is directed to overcoming one or more of the problems set forth above. Briefly summarized, according to one aspect of the present invention, there is provided a method of reading out a CCD interline image sensor having M linesxc3x97N columns of photosites, N vertical shift registers corresponding to the N columns of photosites, and a horizontal shift register, comprising the steps of: exposing the sensor to a first exposure such that all M lines of photosites are exposed; transferring a subset of the M lines of data from the exposed lines of photosites of the first exposure to the N vertical shift registers; serially shifting each line of data from the N vertical shift registers to the horizontal shift register and reading out the horizontal shift register until all of the lines of data have been read out except a second subset of lines of photosites; exposing the sensor to a second exposure such that all M lines of photosites are exposed, while at least a subset of lines of photosites near the horizontal register are xe2x80x9cdoublexe2x80x9d exposed; and transferring a subset of the M lines of data from the exposed lines of photosites (comprising single and xe2x80x9cdoublexe2x80x9d exposed lines of photosites) to the N vertical shift registers; serially shifting each line of data from the N vertical shift registers to the horizontal shift register and reading out the horizontal shift register until all lines of data have been read out except the second subset of lines of photosites, thereby decreasing the frame time of the sensor.
These and other aspects, objects, features, and advantages of the present invention will be more clearly understood and appreciated from a review of the following detailed description of the preferred embodiments and appended claims, and by reference to the accompanying drawings.
The present invention has the following advantages.
1. A significant increase in frame rate of a digital camera without decreasing the image quality.
2. Frame rate improvement results from reducing the number of lines which are clocked out of the imager.